This invention relates generally to semiconductor devices. More particularly, the invention relates to ultra lightweight semiconductor devices and methods for their fabrication. In specific embodiments, the invention relates to ultra lightweight photovoltaic devices.
Weight is a very important factor in many semiconductor device applications. For example, large area photovoltaic generator arrays are frequently used as power sources in aerospace applications where weight is at a premium. The weight of photovoltaic devices is also of significant concern in other mobile applications, and in those particular applications in which transport of massive items is difficult. Specific power is a property of photovoltaic generator devices which is expressed in the units of watts per kilogram of generator weight. Conventional lightweight photovoltaic generators of the type employed in aerospace applications have a specific power rating of approximately 30-50 w/kg. Ultra lightweight photovoltaic generators which are based upon thin film semiconductor materials and which employ very lightweight substrates have been manufactured and such devices exhibit specific powers in the range of 500-1500 w/kg.
Heretofore, such ultra lightweight photovoltaic devices have been relatively difficult to fabricate since they are fabricated from a photovoltaic stock material comprised of a very thin substrate (typically less than 50 microns), having a number of submicron thick layers of semiconductor material supported on the substrate. The processing of such photovoltaic stock material into power generating modules generally requires that the photovoltaic material be cut to size, have current collecting leads affixed thereto, have interconnections established between subunits and that the material be encapsulated in a protective coating. The thin, lightweight nature of the material makes such processing difficult. U.S. Pat. No. 4,754,544, the disclosure of which is incorporated herein by reference, discloses various methods for the fabrication of ultra lightweight photovoltaic generator arrays. As disclosed therein, a first side of a photovoltaic material is protected by a support/encapsulating layer while processing steps are carried out on a second side thereof. Such processing can include etching away some of the thickness of a support substrate to reduce its weight. The methods disclosed therein generally do not permit simultaneous access to both surfaces of a semiconductor device during processing steps; and accordingly, it would be advantageous to have a self-supporting ultra lightweight device which would eliminate the need for a separate support layer and which can provide a device which has increased strength in its end use applications. In addition, any such device configuration and methods should be compatible with conventional semiconductor device processing technology.
As will be explained in greater detail hereinbelow, the present invention provides ultra lightweight semiconductor devices and methods for their fabrication. The devices are capable of being supported without the requirement of an overall support layer. While the drawings, discussion and description presented herein are primarily directed toward photovoltaic devices and methods for their manufacture, it is to be understood that the principles of the present invention are applicable to any type of device comprised of semiconductor material supported upon a substrate member. As such, the present invention also has significant applicability in connection with the fabrication of other large area semiconductor devices such as photo sensors, electrophotographic receptors, large area circuits, memory arrays and the like.
There is disclosed herein a method for manufacturing a lightweight semiconductor device of the type comprising a substrate electrode, a top electrode, and a body of semiconductor material disposed therebetween in electrical communication with the top electrode and the substrate electrode. According to the present invention, a thickness dimension of the substrate electrode is decreased in a first area relative to the thickness dimension of the substrate electrode in a second area so as to define a first and second region of the semiconductor device wherein the thickness of the device in the first region is less than the thickness of the device in the second region. The thicker regions of the device provide support for the thinner regions, while the thinner regions serve to decrease the overall weight of the device.
In specific embodiments of the invention, the device is severed so as to separate the first, relatively thin region of the device from the second, relatively thick region of the device, and such severing generally takes place after at least some of the processing steps are completed. In specific embodiments of the invention in which the thick regions are severed from the thin regions, portions of the top electrode of the device are scribed away so as to delineate a boundary between the first and second regions of the device so that subsequent severing may take place along the scribed portions.
In specific embodiments, the thickness of the substrate may be reduced by etching with acid or alkali in those instances where the substrate is a metal, or by solvent treatment in those instances where the substrate is a polymeric material.
Also disclosed herein are self-supporting, lightweight semiconductor devices manufactured by the methods of the present invention.